1. Field of the Invention
The present invention relates to a technique for automatically measuring the picture quality of a color display which has phosphors in the shape of dots and, more particularly, to a method of measuring the color purity of a color display and an apparatus therefor which are capable of quantitatively measuring the state of phosphors being irradiated with electron beams.
2. Description of the Prior Art
The spot diameter of an electron beam is slightly larger than that of a phosphor, and when the phosphor is appropriately irradiated with an electron beam, the luminance distribution in the phosphor varies concentrically in such a manner as to have a large luminance in inverse proportion to the distance from the center. That is, the luminance at the outer peripheral portion of the phosphor is the same over the entire periphery. However, if the phosphor is not appropriately irradiated, the center of the beam spot and the center of the the phosphor do not agree with each other, and a crescent-shaped non-beam irradiation area is sometimes produced at a part of the phosphor. When a crescent-shaped non-beam irradiation area is produced at a part of the phosphor, the direction and degree of the deviation are known by image processing.
As literatures on color purity measurement, there are the paper by Uno and three others: Automatic Adjustment for CPT Picture Quality (pp. 241 to 244 of Toshiba Review Vol. 36, No. 3 (1981)) and the paper by Nakao and one other: Automatic Measurement for Display Color Purity using Color TV Camera (pp. 49 to 55 of Automatizing Technique Vol. 18, No. 8 (1986)). In these papers, however, the object of measurement is a striped phosphor and it is impossible to apply these techniques to a circular phosphor as they are.
When an electron beam deviates and a crescent area is produced at a part of a phosphor, the direction and degree of the deviation are known by image processing, as described above. However, when the entire range of a phosphor 27 is irradiated with an electron beam 26 in the state in which the spot of the electron beam 26 slightly deviates from the phosphor 27, as shown in FIG. 1(a), the direction and the degree of the deviation are not known, and the deviation produces a problem in the inspection of a high-precision color display tube.